1. Field of the Invention
The present invention relates to superconductors having a high transition temperature and manufacturing processes thereof.
2. Related Art Statement
Heretofore, the study of materials with a high critical temperature (Tc) of the transition from a normal conducting state to a superconducting state has been a very active research areas. In 1973, J. R. Gavalar sputtered Nb-Ge films in high argon pressure onto hot substrates and obtained superconducting transition with an onset temperature (Tco) of 22.3.degree. K. (Appl. Phys. Lett. 23, 480, 1973). Soon after that, L. R. Testardi achieved an onset temperature (Tc) of 23.2.degree. K. with the same material (Solid State Commun. 15, 1, 1974). Many researchers extensively have examined Nb.sub.3 Ge and, however, the Tco for this compound has not exceeded 23.9.degree. K. until now.
D. Hughes, et al predicted from the empirical relationship for the critical temperature of some A 15 superconductors, that if Nb.sub.3 Si crystallized in the A 15 structure, it should have Tc.apprxeq.38.degree. K. (Nature, 250, 723, 1974). However the crystal in a good ordered state with a small lattice constant is very difficult to make. S. Geller also suggested a stoichiometric .beta.-W type Nb.sub.3 Si should have a superconducting transition temperature of between 31.degree. K. and 35.degree. K. (Appl. Phys., 7, 321, 1975). Many experimental approaches have been tried. Although a new phase prepared was identified as A 15 and lattice spacings being as low as .about.5.09 .ANG. that was almost the predicted value, the Tc for Nb.sub.3 Si still remained to be 18.6.degree. K.
Thus, many efforts to raise the critical temperature (Tc) have so far been made, nevertheless, it has increased at a rate of only about 1.degree. K. per year, and the presently known highest superconducting critical temperatures do not exceed 23.6.degree. K.
Accordingly, in order to utilize those conventional superconductive materials as a superconductor, it is necessary to cool them with a cryogen having a freezing temperature not higher than 23.6.degree. K.
Freezing temperatures of known cryogens are enumerated below.
Liquid helium; 4.2.degree. K. PA1 Liquid hydrogen; 20.4.degree. K. PA1 Liquid air; 70.degree.-80.degree. K. PA1 Liquid nitrogen; 77.degree. K.
Among the above, liquid helium, which is a scantly natural resource has so far been used as a cryogen for developing superconductivity, because the critical temperatures of conventional superconductive materials are relatively low.
Accordingly, superconductive materials with a high critical temperature (Tc) has so far been eagerly desired to be developed.